A number of techniques exist for applying metallized contacts or conductors to the front and back surfaces of a solar cell substrate. Unfortunately, these known techniques typically suffer from one or more drawbacks which limit their utility. Such drawbacks include excessive material costs, unreasonably long application times, lack of uniformity of thickness of the metallized contact, and excessive breakage rates of the solar cell substrates as a consequence of application of the metallization.
One such known technique, commonly identified as screen printing, involves applying a screen having a metallization pattern formed therein to one side of a substrate. A metal screen printing ink is then spread over the metallization pattern in the screen and onto the surface of the underlying substrate using a narrow elongate blade which is moved across the screen in direct contact therewith. After removing the screen the metallized ink is fired to drive off the binder in the ink and cause the metal in the ink to adhere to the substrate. Such screen printing methods are described in U.S. Pat. Nos 4,293,451 and 4,388,346.
Screen printing is an effective technique for applying metallized contacts to certain types of solar cell substrates. Unfortunately, screen printing suffers from several drawbacks when used to apply metallized contacts to solar cell substrates which are relatively brittle and have irregular, uneven surfaces. First, application of metallized inks by screen printing to EFG-grown solar cell substrates often results in significant variation in the thickness of contacts formed from the metallized inks. This variation in thickness is caused by the surface configuration of EFG substrates. Typically the surfaces of EFG-grown substrates have undulations or random peaks and depressions with a flatness deviation in the range of 4 to 10 mils. Because the printing screen rests on the high points of an uneven or irregular substrate surface, the thickness of the metallized contacts formed by the screen printing process may vary significantly over the width and length of the metallized contact. Such variation in thickness can result in the excessive use of metal printing ink, thereby adding to the overall cost of the solar cell. Additionally, if the metal ink is applied in a thickness greater than that required for satisfactory electrical current flow, as occurs with those portions of the screen printed contacts overlying the low spots of the substrate, the substrate may tend to bow as a result of stresses caused by the firing process which bonds the metal ink to the substrate Such bowing is disadvantageous because it can lead to cell breakage and can make the attachment of discrete solar cells to a large solar cell array problematic.
A second problem with applying metallized contacts to a brittle solar cell substrate having uneven surfaces by screen printing is that an unacceptably large number of substrates typically break as a consequence of the process. Such breakage is believed to occur due to the relatively large forces applied, as measured on a per unit of surface area basis, to the substrate by the narrow blade used to spread the metal ink across the screen.
Techniques such as spraying or evaporative deposition may be used to apply metallized layers having a uniform thickness to a solar substrate. Unfortunately, such techniques involve masking and other limitations. Photolithography may also be used to apply metallized layers in the form of a pattern, such as that of a front surface grid electrode for a solar cell. However, photolithography adds to the time and cost of producing a solar cell substrate.
In fields of technology unrelated to the production of solar cells, it is known to apply patterns to objects having an irregular surface by a technique commonly identified as pad printing. For instance, pad printing is used to apply decorative patterns to golf balls, watch faces, china, glassware and toys. Conventional pad printing devices typically include a gravure plate having an etched-out portion corresponding in configuration to the pattern to be printed etched therein, a workpiece support, an ink applying pad, and a blade support having a flood blade and doctor blade attached thereto. The pad and blade assembly are attached, in mutually-spaced relation, to a movable carriage. After attaching a workpiece to the workpiece support, printing ink is poured onto the surface of the gravure plate. Then, the flood blade is drawn across the gravure plate so as to spread printing ink into the etched portion of the plate. The doctor blade is then used to remove ink from all but the etched portion of the gravure plate. The pad is then (1) lowered into contact with the gravure plate to pick up the ink on the etched portion of the gravure plate, (2) raised and moved over the workpiece, (3) lowered into contact with the workpiece, and (4) raised up away from the workpiece, leaving an ink pattern printed on the workpiece.
Conventional pad printing processes (i.e., pad printing applied to commercial articles such as china, glassware, golf balls, watch faces, toys, etc.) have certain characteristics which render the processes unsuitable as a method of applying metallized conductors to a solar cell substrate. The thickness of a pattern applied by conventional pad printing processes typically is about 0.25 to about 1.0 mils thick when the ink is still wet, and ranges from about 1.0 to about 2.0 microns after the ink has been fired. Solar cells, on the other hand, require metallized conductors having a thickness of at least about 4-10 microns after firing. Also, known pad printing techniques are not believed to involve the use of conductive metal inks to form electrical conductors. Further with regard to prior known pad printing techniques, with time the surface of the gravure plate and the working edge of the doctor blade often become nicked or scored, with the result that stray ink deposits are picked up by the pad and deposited so as to extend away from the printed pattern. With solar cells, such stray ink patterns could be problematic, depending upon their placement, since they may electrically couple the front and back sides of the solar cell, thereby destroying isolation of the p/n junction.